A communication device such as a personal computer, or a mobile telephone having a packet communication function, is able to download various types of programs downloaded from a server connected to the Internet.
While an open network such as the Internet enables people worldwide to exchange programs freely, it also has inherent risks, including, for example, data theft from a communication device. Also, a program which causes a malfunction in a communication device may be provided without malicious intent.
Accordingly, a problem might occur if a resource inside or outside a communication device is accessed easily by a program, provided through a network, without any restrictions. For example, a user's telephone number, mail address or bank account number, each of which is stored in a communication device, may be freely read out from the communication device. In view of these risks, user's privacy is a major concern.
It is possible to restrict the functions of programs provided to communication devices. For example, a mobile phone which is able to execute programs written in Java® imposes restrictions on such programs. Specifically, programs are only authorized to access the restricted resources, and programs are not authorized to access resources such as address book data or user's personal information.